The invention relates to exhaust aftertreatment filters for filtering exhaust from internal combustion engines, including diesel engines, and more particularly to regeneration of such filters by heat to incinerate or burn-off contaminant particulate collected from the engine exhaust.
Exhaust aftertreatment filters for diesel engines are known in the prior art. The filter traps contaminant particulate in the exhaust. The filter is composed of regenerable material which is regenerated by heat to burn-off the trapped contaminant particulate. These filters can become plugged if conditions necessary for regeneration of captured particulate such as soot are not achieved. Such conditions typically occur in stop-and-go city driving conditions and extended periods of idle and/or low load. In such situations, exhaust temperatures are not hot enough to trigger incineration of captured diesel particulates in the filter. To overcome this problem, heat can be applied in a variety of ways. In the past, emphasis has been on heating the entire filter to regenerate it. This requires significant energy consumption. Furthermore, in the process, heat is not always efficiently utilized, and filter durability issues can result.
The present invention addresses and solves the above-noted problems, including energy consumption and durability issues. The entire filter is not necessarily heated, but rather localized heating at strategically chosen locations is instead recognized and used. Contaminant particulate tends to collect in the ends of the filter, particularly the downstream end. Heating elements are accordingly located at points along the axis of the filter where particulate accumulation is greatest and where heat application and regeneration have the greatest affect. An advantage of localized heating is that energy can be focused at specific points along the filter, and, if needed, regeneration can be initiated at different locations at different times, to conserve energy. There is no need for additional heating elements nor for heating the entire filter element.
In one aspect, heating is applied across radial cross-sections of the filter, and the axial location of these cross-sections is determined based on where particulates are expected to accumulate. This is significant in that there is regeneration uniformly across the cross-section of the filter, in contrast to prior methods characterized by radially distributed failure patterns due to uneven heating across the cross-section. One or more cross-sectional heating elements may be used in a particular filter element.
In another aspect, axially aligned conductors are used to facilitate flow of electrical current and/or thermal energy. When multiple cross-sectional heating elements are used, the axial conductors typically conduct both electricity and heat. In single cross-sectional heating element versions, the axial conductors may be used solely as heat conductors and not to conduct electrical current.
The geometry and method of manufacture of the filter element are significant. The filter element is spiral wound by rolling layers of flat and pleated sheets into a roll. The process and geometry allows the heating element conductors to be easily incorporated into the media and form cross-sectional heating conductor elements with uniformly spaced electrically and/or thermally conductive material. This is not possible with extruded filter elements such as cordierite monoliths. The process also allows heating elements to be interconnected by axially aligned conductors or to be individually or directly attached to a power source.
In another aspect, the conductors used as heating elements serve a dual function, namely firstly as electrical conductors, and secondly as heat conductors to conduct heat to other portions of the filter. The latter is important when conductors are aligned axially to transfer heat from the strategically heated locations to other portions of the filter.
In a further aspect, the electrical and/or thermal conductors are embedded into the filter media and/or attached to the surface of the media with a suitable binder or adhesive or are laminated in place. The conductors are oriented axially and/or laterally. The axial location of the laterally extending conductors is significant. It is preferred that the first such conductor be located as near as possible to the edge of the filter media as it is spiral wound, to provide such conductor located at the axial end of the filter roll after such winding. Other laterally extending conductors are axially spaced at intervals along the media as determined by heating needs. For electrically heated filters, these would typically be spaced at regular intervals along the entire upstream to downstream axial length of the filter roll.
In a further aspect, electrical and/or thermal conductors are additionally provided which are oriented and extend axially at laterally spaced intervals. This can further enhance thermal efficiency.
In a further aspect, two sheets of media are spiral wound to form the filter roll, one sheet being flat and the other being pleated. When sets of both axial and lateral conductors are used, it is preferred that the set of laterally extending conductors be provided on one layer, and the set of axially extending conductors be provided on the other layer.
The conductors may be in various forms, including round wire, flat ribbon, particle based bound into adhesive or a binder, and the like.
In a further aspect, the heating elements are not built into the media nor rolled therewith, but rather are attached to the end of the filter. The heater element is energized by direct connection electrical resistance heating. The heater element conducts thermal energy to the filter element.
In a further aspect, microwave energy is coupled to the filter element via a waveguide or an antenna, and the filter is heated at strategic locations for faster regeneration. Since the heating rate is proportional to the microwave power supplied, it will take a substantial amount of microwave power to provide uniform heating of the entire filter element. It is thus important to use the energy to heat the filter at the areas where it is most needed for faster regeneration. The most effective way is to create a hot zone by strategically placing the microwave emitter (e.g. antenna or slotted waveguide) where the highest concentration of soot or other contaminant particulate is located. Waveguides or antennas are placed at one or both ends of the filter, and can be internal or external to the filter element. In one aspect, slotted waveguides are placed within the filter housing externally of the filter element and near the axial ends of the filter. When slotted waveguides are used on the upstream dirty side of the filter, care must be taken to keep the soot particles from entering the microwave power system, as this will degrade or damage same. The waveguide on the downstream clean side is protected from the pollutant and is therefore at less risk. Antenna probes can conduct microwave energy to heat the regions near both ends of the filter. The antenna probe can be cylindrical or with a doorknob or ball shape, which allows for higher power levels without arcing.
In further aspects, the waveguide or antenna is located within the filter between the upstream and downstream distally opposite axial ends of the filter element. A center core is cut out in the filter, and the area is dependent on the size of the waveguide or antenna. The geometry of the waveguide or antenna is designed such that the energy distributed is at the highest near both ends of the filter. This may be accomplished by using uniformly spaced slots in the waveguide or a shaped antenna.